Field Trials of Aerospace in Mechanical and Structural Applications

George Nadim Melhem School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052 Australia: Perfect Engineering Pty Ltd, 14 Dick St, Henley, NSW 2111 Paul Richard Munroe and Charles Christopher Sorrell School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia Alsten Clyde Livingstone School of Civil and Environmental Engineering, University of Technology Sydney, Ultimo, NSW 2007, Australia; Perfect Engineering Pty. Ltd., 14 Dick Street, Henley, NSW 2111, Australia

Abstract The present work reports findings for the application of specialized aerospace aluminum rivets, manufactured from Al 7075 (Al-Zn-Mg-Cu) T6 stem/, with an Al 5056 (Al-Mg) shank or sleeve, which were used for construction rectification of an outdoor louver façade on a high-rise building. These specialized rivets were used to replace failed conventional construc- tion rivets, which consisted of sleeve and mandrel comprised of either all-, all-aluminum, or aluminum-steel. The building is in close vicinity to the ocean and exposed to extremely high wind loading, making the rivets susceptible to failure by corrosion and fatigue. The focus of the present work is to report the examination of the specialized replacement rivets following an in-service lifetime of 12 years. The examination revealed that the replacement rivets (mandrel and sleeve) remained intact and uncontaminated, essentially free of corrosion. It is likely that sunlight exposure and the composite nature of the rivets enhanced the performance through age hardening. Analysis of the rivets included visual inspection, optical microscopy, Vickers microhardness testing, and transmission electron microscopy. The aim of the analysis was to correlate microstructures with microhardnesses, using these data to evaluate the ultimate tensile strength (UTS), yield strength (YS), and the potential for further age hardening. The Vickers microhardnesses were observed to have increased by ∼8% over the service lifetime of 12 years, which equates to increases in YS (34.8–46.8 MPa) and UTS (23.8–45.6 MPa). Although the results show that there is a large increase in the strength values when comparing the unused rivets to the 12-year-old rivets, this increase in hardness may not necessarily be due purely to natural aging kinetics such as exposure from the sun and outdoor temperature. However, there appears to be some insignificant alteration of the microstructure and mechanical properties as a result of this exposure.

Keywords: Aluminum alloys, 5056; 7075; Age hardening; Hardness testing; Microscopy. Extrudable Al-Si-MG–Hall–Heroult Al-Si-MG–Hall–Heroult

INTRODUCTION treated, the YS of age-hardenable alloys can increase owing to: (1) (the primary mech- Background to Aerospace Aluminum Rivets anism for strengthening of aluminum alloys), (2) solid solution hardening or strengthening, (3) strain hardening Pure aluminum in the annealed condition has a low yield or work hardening, and/or (4) refinement of the grain size strength (YS) of 7–11 MPa.[1] However, the addition of or grain structure (grain boundary strengthening).[2,4] small amounts of specific alloying elements can increase The combination of low cost, lightweight, ductility, high the YS to 200–600 MPa or more.[1] Age hardening also strength, and toughness makes age-hardenable alloys can enhance the YS as high as 450–600 MPa.[2] There suitable for uses as structural and semistructural parts in are two major groups of wrought aluminum alloys, which aircraft.[5] The Al 7000 series alloys are used commonly in are age hardenable and non-age hardenable.[3] When heat a wide range of applications in aerospace.[2]

Encyclopedia of Aluminum and Its Alloys, First Edition DOI: 10.1201/9781351045636-140000309 Copyright © 2018 by Taylor & Francis. All rights reserved. 1007 1008 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

Table 1 Mechanical data for aluminum alloys used commonly in aerospace rivets[11] Aluminum alloy Tensile strength Shear strength Fatigue strength grade Temper (MPa) YS (MPa) (MPa) (MPa) References 1100 O 90 34 62 34 [12] H14 124 117 76 48 H18 165 152 90 62 2017 T4 427 276 262 124 [13] 2024 T3 483 345 283 138 [13] 2117 T4 296 165 193 97 [14] 2219 T851 455 352 285 103 [13] 5056 O 290 152 179 138 [12] H18 434 407 234 152 H38 414 345 221 152 7050 T7451 524 469 303 240 [13, 15] 7075 T6 572 503 331 159 [9]

Current Applications of Aerospace Rivets in aircraft, the member components must be deconstructed, Mechanical and Structural Applications defined, and quantified so that their interactions both within the member and between members are understood. In an The use of rivets in aerospace construction is a well- aircraft, there are many engineering variables that repre- established practice.[6–9] The first use of aluminum rivets in sent major challenges to the designer in classifying the vast aircraft was in 1936 in the United States by Cherry Aero- amount of materials employed according to specific grade space.[10] Table 1 gives some of the mechanical properties and type of materials, physical and chemical characteristics, of aluminum alloys that are used commonly in aerospace joining of materials, ensuring their compatibility to mitigate rivets. galvanic corrosion, fluctuating loads, and other variables. Rivets are used widely to join materials used in aerospace applications. These rivets generally are equivalent in chem- Background to Refurbishment Using ical composition and mechanical properties to the materi- Specialized Rivets in Façade: Design als they join together. An industry rule of thumb is that the and Construction number of blind rivets used in an aircraft is five blind riv- ets to three solid rivets of the same diameter.[16] Although a Rivets are used not only in the construction of aircraft blind rivet is not as strong as a solid rivet, it has clear advan- but also routinely in the building industry. Although it is Al-Si-MG–Hall–Heroult tages in ease of installation over other types of fasteners, and apparent that aerospace rivets offer superior performance [17]

Extrudable Extrudable it is not susceptible to vibrational loosening. Fig. 1 illus- in terms of corrosion resistance and mechanical stability, trates the aerospace rivet used in the present work. Fig. 2 illustrates a cross section of an installed rivet and some of 21 the characteristics deriving from its design. Fasteners are an important integral component in the 3 assembly of an aircraft structure, and there are thousands Material of fasteners and rivets used in the construction of most air- AA5056 [18,19] craft. However, in order to understand how fasteners AA7075 or rivets affect the performance of specific structures of an Riveted material 4 Pin or Sleeve 5056 Mandrel 7075 aluminum aluminum alloy alloy

Fig. 2 Cross section of installed rivet and characteristics: 1 = Flush installation makes easier to paint and resistant to salt and water, 2 = Flush pin break eliminates need for grind- ing/filing, 3 = Solid-circle lock ensures maximal strength and resistance to vibration—designed to resist pin pushout, 4 = Sleeve expands during installation, tightly filling hole to create a Fig. 1 Aerospace rivet used in present work[17] weather-resistant joint Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1009 their use in structural applications remains limited. This 5056, are not affected by heat, but they can be strength- is likely a result of limited experience with these products ened by work hardening or cold working, resulting in YS and, more importantly, higher costs. 150–405 MPa.[23] The present work reports rectification done on an When sodium chloride is present in the environment, external roof façade in which the existing rivets had alu- crevice corrosion, where water and corrosive contaminants minum sleeves and steel , the latter of which had remain trapped in narrow gaps, may occur. Since rivet corroded and degraded mechanically such that the load design incorporates such internal gaps, the risk of this type was carried by the aluminum sleeve and so failure of the of corrosion is inevitable. Sealing or may help louver system occurred. The rectification by Melhem et reduce this potential. al.[11] involved the use of an Al 7075 rivet pin with an Al 7075 is susceptible to embrittlement due to micro-

Al 5056 sleeve. The rivets were used to join aluminum segregation of MgZn2 precipitates, which leads to cat- louver mullions and profiles of grades 6060, 6063, and astrophic failure.[24] Simultaneously, its resistances to steel brackets on a multistorey building. The work was oxidation and pitting corrosion are reduced in corrosive completed in June 2005, and the installation has been environments.[25] Consequently, Al 7075 in the T6 condi- monitored continuously since then. More than 10,000 tion should not be used in aggressive environments unless rivets were used in this work to join the louvers as well protected by anodizing, chromate coating, or painting. as custom-designed aluminum sections that consisted of The distribution of second-phase material has a signif- 6063, 6060, 6061, and Al 6351. Al 7075 rivet pin (with icant influence on the corrosion behavior of high-strength gold chromate coating) with Al 5056 sleeve (with clear aluminum alloys. If second phases are located preferen- chromate coating), with 3/16″ (4.73 mm) diameter, was tially at grain boundaries, they may promote intergranu- utilized since these materials are compatible from a gal- lar corrosion (IGC) owing to their compositions and hence vanic perspective for the joining of the aluminum façade half-cell potential differences with the adjacent alloy structures on the building. Furthermore, Table 1 reveals matrix.[26] Furthermore, the narrow band on either side of that the rivets used have exceptionally high tensile and the grain boundary, which is a precipitate-free zone (PFZ), shear properties. also influences the corrosion behavior of aluminum alloys since the PFZ is depleted of alloying elements, again estab- Corrosion lishing half-cell potential difference. Consequently, IGC of high-strength aluminum alloys often is attributed to com- In general, corrosion rates of single-phase aluminum alloys positionally different features at the grain boundary and are low.[20] However, the corrosion rate of aluminum alloys associated anodic dissolution of (1) the PFZ, where noble increases once a second phase has precipitated. The contin- alloying elements, such as copper, are depleted; (2) anodic ued growth of the precipitates suggests that their eventual second phase precipitates at the grain boundaries; or (3) coalescence would result in a decrease in corrosion owing segregated alloying elements, such as magnesium or impu- to alteration overall decrease in the surface area of the rity elements, at the grain boundaries. In the Al 7000 series, galvanic cell. In fact, some alloys exhibit maximal corro- in which anodic precipitates, such as MgZn2, are formed sion when in the overaged condition.[20] Furthermore, cold at the grain boundaries, these precipitates are relatively working can enhance corrosion since the metal is highly active in relation to the alloy matrix,[26] so IGC occurs. The stressed, thus generating dislocations that promote the associated precipitation occurs as both heterogeneous and [20]

heterogeneous nucleation of precipitates during aging. homogenous. In the former, the YS increases at the grain Extrudable Also, the Al 5000 series of non-heat- treatable aluminum boundaries or isolated precipitation sites, so this type of alloys is more susceptible to corrosion cracking precipitation does not contribute to strengthening of the Al-Si-MG–Hall–Heroult when cold worked.[21] grains. In the latter, the precipitates in the grains are coher- It is clear that the susceptibility to environmental cor- ent or semi-coherent, so they exert minimal strain, thereby rosion depends on a range of factors, which included heat allowing hardening within the grains to occur. treatment and mechanical working, chemical composition, mineralogical composition, microstructure, and envi- ronmental factors (e.g., temperature, wind fatigue). Cor- EXPERIMENTAL PROCEDURE AND rosion usually is initiated by the presence of rainwater, OBSERVATIONS condensed humidity, seawater, and/or sea spray, which can initiate electrochemical reactions due to anodic corrosion Site Investigation or galvanic corrosion. However, as indicated previously, heat-treatable aluminum alloys (e.g., Al-Zn-Mg-Cu), The present work is based upon a case study that was such as Al 7075, may undergo changes in their properties conducted initially in early January 2005 on a high-rise continually with time, depending on a number of factors, building close to the coast in Sydney, Australia. Gener- resulting in YS as high as 500–505 MPa.[2,22] In contrast, ally, locations within 8–16 km of saltwater are considered non-heat-treatable alloys (e.g., Al-Mg-Mn), such as Al coastal since the wind can carry the salt much further than 1010 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

16 km. [27] It had experienced a major corrosion failure of (Level 29) and façade (Levels 26–28) was completed in the roof and aluminum louver façade in June 2005. The building has been inspected regularly in August 2003. The failure occurred on Level 29, which was order to confirm that the rectification remained both intact the roof top, owing to extreme wind speeds in the 100–120 and corrosion-free. The last inspection was carried out on km/h range. The roof and façade failed, as indicated in March 29, 2017. At that time, several of the Al 7075/Al 5056 Fig. 3, which resulted in debris falling on the south aspect rivets were extracted from various locations around the into a childcare center and public footpath. This occurred building. Replacement rivets were identical to those used on the weekend, and there were no injuries or fatalities. in the original rectification. Visual inspection revealed no The failure occurred well before the design life of obvious damage or apparent corrosion to either the rivets the building. Examination of the failed sections revealed or aluminum structures. As suggested in Fig. 4, the rivets extensive corrosion, largely owing to design flaws in the appeared to be in pristine condition. These images may be form of joining aluminum louvers to structural steel. The contrasted with the appearance of the structure following façade louvers failed from galvanic corrosion as the louver rectification in June 2005, shown in Figs. 5–7. mullions and profiles were joined with rivets with Al 5056 aluminum sleeves and steel mandrels. Chemical analyses Wind Load on Structure by inductively coupled plasma atomic emission spectrome- try revealed that the mullion was made of Al 6063 and the Despite the high wind speeds to which the structure was louver was made of Al 6060. subjected, the wind loading imposed and translated to the The replacement of the existing steel/aluminum riv- rivets was calculated to be quite low and within an accept- ets with aerospace Al 7075/Al 5056 rivets in the rooftop able range to withstand failures or pullout of the rivets.[28]

Damaged louvres section facing south and the Al-Si-MG–Hall–Heroult childcare centre is directly below. Extrudable Extrudable

Fig. 3 Damaged louver section on southwest corner of roof on Level 29

(a) (b)

Fig. 4 (a) Photo taken on March 29, 2017 of Al 6061 flat strip riveted louver structure on Level 28, showing rivets still in pristine condition; (b) photo taken on March 29, 2017 of Type A angle, showing intact and clean rivets Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1011

Fig. 5 Photo taken in June 2005, showing original Al 6061 flat strip riveted louver structure and two mullion sections connected and secured on Level 28 (identical to Level 29)

(a) (b) Rivet Installation

Although the wind loading may have had a small effect Type “A” on the sleeves by subjecting them to modest mechan- angle ical forces, it is likely that most of the work harden- ing occurred in the sleeve surface when the rivet was installed. As shown in Fig. 8, the rivet mandrel is pulled by the rivet gun through the sleeve, but the mandrel head is of wider diameter than the sleeve, thereby expand- Fig. 6 (a) Original louver system on Level 28; new Type A Al ing the tail. These actions force together the two sheets 6060 aluminum angle riveted in June 2005; (b) schematic design being joined and clamp them tightly. The serrations on of angle connection Type A the mandrel are gripped tightly by the rivet gun, which is positioned directly on the workpiece. This prevents any (a) (b) movement, vibration, or pin pushout. With further pulling by the rivet gun, the mandrel is snapped at the groove. The most successful aerospace rivet probably is the Huck Magna-Lok®, which is manufactured by Alcoa Fastening Type”B” Systems.[29] angle

Sample Acquisition Extrudable Thirty-two rivets were extracted from diverse locations in Al-Si-MG–Hall–Heroult Al-Si-MG–Hall–Heroult order to achieve a representative sampling of the 12-year- Fig. 7 (a) Original louver system on Level 26; new Type B Al 6060 aluminum angle riveted in June 2005; (b) schematic design old rivets exposed to the four cardinal directions as shown drawing of angle connection Type B in Figs. 9 and 10. Approximately half of the rivets were drilled from the head section of the rivet, and the other half However, wind loading is complicated in that it may involve were drilled from the blind section in order to be able to a combination of tensile, compressive, and shear stresses. examine the non-drilled sections for the entire rivet length. This is particularly the case since the wind pressure for A 3-mm-diameter drill bit was used for this. the building shape was considered to include both positive (wind blowing directly on the building, thus creating for- Sample Selection ward pressure) and negative (wind passing the building, thus creating a negative back pressure). The fluctuating Two typical rivets in service for ∼12 years (Samples 1 and 2) wind loads were calculated for the louver structures during were selected for extended analysis. A new rivet (Sample 3) the initial investigation according to Australian Stan- was purchased from a supplier under the assumption that it dard AS 1170 Part 2–2002. Minimum Design Loads on had been manufactured relatively recently. All rivets were Structures, Part 2: Wind Loads. comprised of Al 7075 sleeve and Al 5056 mandrel. 1012 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

Tool

Serrated mandrel head

Rivet head Rivet shank Rivet tail Mandrel head Fig. 8 Schematic of rivet system[11] (based on operation of Huck Magna-Lok®[29])

(a) (b)

Fig. 9 (a) Extraction of 12-year-old rivets from head section in March 2017; (b) extraction of 12-year-old rivets from blind section in March 2017

Mounting unit was used to prepare the resin holding the sectioned rivet, and this was prepared with 1 part hard- ener to 4 parts epoxy. The resin solution was stirred by hand until it was sufficiently transparent. Samples were placed inside a vacuum pump for 5 min so that any excess

Al-Si-MG–Hall–Heroult air bubbles would be removed. The prepared samples were left overnight to harden in the resin moulds, and Extrudable Extrudable the samples were coated with a thin layer of vaseline for lubrication and to aid in easy extraction from the mould. Four levels of grinding were carried out using the success- fully finer silicon–carbide pads of the following grit levels or grades: 120, 320, 1200, and 2500. Grinding was carried out at low speeds of 200 rpm on a Struers Labopol 5 until the sample surface was coplanar. The samples were then cleaned thoroughly prior to polishing, and this was achieved by care- Fig. 10 Installation of new replacement rivets where 12-year- fully rubbing the surface with cotton wool under running old rivets were extracted in March 2017 water, followed by immersion in a soapy water mixture in an ultrasonic bath for 2 min. Samples were dried by the addition Sample Preparation of ethanol to the sample surface, followed by blow-drying using compressed air. Polishing was carried out on a Struers The rivets were prepared by sectioning with a Struers Labopol five at a low 150 rpm. Polishing began at Minitom diamond saw, which uses cooling oil. They then 3 micron (μm) using 3 μm oil-based polycrystalline Kemet were cold mounted and polished by standard metallo- Diamond suspension on a Kemet NSH-BM polishing pad, graphic preparation methods. Cold mounting is used in followed by 1 μm using 1/10 μm oil-based polycrystalline order that the temperatures due to hot mounting (similar Kemet Diamond suspension. The polishing process was fin- to aging) and any other thermal influence that alters the ished using oxide polishing-colloidal silica suspension and a grain boundaries are completely avoided. A Buehler Cold Kemet Chem-HM polishing pad. The polished surface was Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1013 checked between polishing stages on an Olympus PMG 3 microscope at 25× magnification. 2.10 The polished surface was checked prior to etching on a 2.1 2.12 2.13 × 2.2 Nikon Epiphot 200 microscope at 100 magnification. The 2.11 samples were etched with Keller’s Reagent (1.5 mL HCl,

1.0 mL HF, 2.5 mL HNO3, 95 mL distilled water), where 2.9 2.14 the samples were immersed in the solution for 30 s prior to 2.3 2.7 Key being cleaned by ethanol. The etching process was carried 2.8 2.15 AA5056 out under a fume hood at room temperature. 2.4 Figures 11–13 show a schematic of the cross sections 2.5 2.6 AA7075 of the samples that were sectioned and prepared using a diamond blade for analysis. The nomenclature uses the formalism in which the first number refers to the sample Fig. 12 Sample 2—Longitudinal cross section of 12- year-old (1, 2, or 3) and the second number identifies the location rivet, showing mandrel on inside, sleeve on outside, and locations of the analysis. for microhardness testing

Testing 3.13 Vickers Microhardness 3.14 3.12 3.3 The Vickers microhardness measurements were performed 3.2 using a Stuers Duroscan G5 with a weight of 300 g applied 3.7 3.1 3.15 3.11 3.4 Key 3.6 3.5 3.10 AA5056 3.8 AA7075 3.9 1.8 1.16 1.10 1.9 1.15 Fig. 13 Sample 3—Diametral cross section of new rivet, showing mandrel on inside, sleeve on outside, and locations for microhardness testing 1.17

1.14 for a dwell time of 15 s. Indent images were recorded using 1.4 a Nikon Epiphot 200 microscope. These data are given in Table 2 and they are summarized 1.3 in Table 3. 1.2 Extrudable

Table 2 Locations (Figs. 11–13), Vickers Microhardnesses Al-Si-MG–Hall–Heroult 1.1 1.11 1.13 (VHN), and alloy types Sample location VHN Alloy 1.1 117 5056 1.2 113 5056 Key 1.3 114 5056 1.7 AA5056 1.4 108 5056 1.6 1.12 1.5 96 5056 1.18 AA7075 1.5 1.6 113 5056 1.7 106 5056 1.8 105 5056

Fig. 11 Sample 1—Longitudinal cross section of 12-year-old 1.9 107 5056 rivet, showing mandrel on inside, sleeve on outside, and locations 1.10 200 7075 for microhardness testing (Continued) 1014 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

Table 2 Locations (Figs. 11–13), Vickers Microhardnesses Correlations of Vickers Microhardnesses with YS (VHN), and alloy types (Continued) and Ultimate Tensile Strength Sample location VHN Alloy 1.11 175 7075 The Vickers microhardness can be converted to YS and ultimate tensile strength (UTS) through the use of empiri- 1.12 181 7075 cal equations. Equations were created from data provided 1.14 186 7075 by Pagliarello.[30] Equations devised by Clark et al.[31] for 1.15 180 7075 altered 7075-T6 specimens were also used. The empirical 1.16 120 5056 equations are shown in Eqs. 1–5, where Eq. 1 provides 1.17 115 5056 the conversion of ‘Vickers Hardness Number’ (VHN) 1.18 114 5056 hardness to ‘Hardness-Rockwell B’ (HRB) (All of these approaches require conversion of the Vickers microhard- 2.1 108 5056 ness to the Rockwell B hardness, which was done using the 2.2 108 5056 data of ISO 18265:2013 Metallic Materials—Conversion 2.3 103 5056 of Hardness Values), Eqs. 2 and 3 provide the calculation 2.4 116 5056 for YS and UTS, respectively, by Pagliarello,[30] and Eqs. 4 2.5 179 7075 and 5 provide the calculation for YS and UTS, respectively, [31] 2.6 182 7075 by Clark et al.; 2.7 180 7075 2.8 183 7075 =×+() ≤≤ HRB 0.3HHvv 34.987 140 195 , 2.9 174 7075 using the data ofISO 18265 : 2013 2.10 185 7075 Metallic Materials: Conversion of 2.11 187 7075 Hardness Values) (1) 2.12 104 5056 2.13 106 5056 YS=× 9.75 HRB − 388.19,[]31 (2) 2.14 105 5056 2.15 111 5056 3.1 170 7075 UTS=× 9.49 HRB − 273.64,[]31 (3) 3.2 169 7075 3.3 168 7075 =× − []32 3.4 170 7075 YS 8.92 HRB 309.95, (4) 3.5 166 7075 Al-Si-MG–Hall–Heroult 3.6 167 7075 UTS=× 6.11 HRB + 0.76.[]32 (5) Extrudable Extrudable 3.7 167 7075 3.8 104 5056 3.9 106 5056 The results of these conversions are given in Tables 4 3.10 104 5056 and 5. It can be seen that the data for YS agree relatively well, whereas the data for UTS are in less agreement. 3.11 105 5056 Table 6 contrasts UTS values calculated on the basis 3.12 101 5056 of Pagliarello and Clark et al.[30,31] using data from other 3.13 104 5056 researchers.[32–35] Table 7 provides further contextual infor- 3.14 105 5056 mation in the form of experimental data for the mechanical 3.15 106 5056 properties of interest.

Table 3 Averages and standard deviations of VHN values Standard deviation Standard deviation Sample VHN (Mandrel 7075) VHN (Sleeve 5056) (Mandrel 7075) (Sleeve 5056) 1 184 111 7.80 6.25 2 181 108 3.96 3.97 3 168 104 1.46 1.49 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1015

Table 4 Estimations of YS (MPa) for Al 7075 σ [30] σ [31] Sample Average Hv Conversion to HRB Pagliarello ( y) Clark et al. ( y) 118490.2491.3494.6 218189.3482.5486.6 3 168 85.4 444.5 451.8

Table 5 Estimations of UTS (MPa) for Al 7075 σ [30] σ [31] Sample Average Hv Conversion to HRB Pagliarello ( UTS) Clark et al. ( UTS) 1 184 90.2 582.4 551.9 2 181 89.3 573.8 546.4 3 168 85.4 536.8 522.6

Table 6 UTS values for Al 7075 calculated from other work Hardness σ σ [30] σ [31] Condition ( UTS) VHN HRB Pagliarello ( UTS) Clark et al. ( UTS) Reference T6 606 — 92 599.4 562.9 [32] T6 630 170 — 547.2 529.3 [33] T6 603 — 86 542.5 526.2 [34] T6, T651 572 175 — 560.5 537.8 [35]

Table 7 Experimental UTS (MPa) and YS (MPa) of Al 7075 Transmission Electron Microscopy reported by others σ σ Specimen Preparation Condition ( UTS)(y) Reference

T6 510–538 434–476 [36] Transmission electron microscopy (TEM) samples of the Al T6, T651 572 505 [35] 7075 mandrel were prepared by focused ion beam micros- T6 572 518 [37] copy using the external lift-out method[38] using an FEI T6 (20 years old) 574 523 [37] Nanolab 200. The surfaces of the specimens were protected using in situ platinum deposition prior to application of the ion beam. The sample locations were adjacent to those desig- nated as 2.7 and 2.8 in Fig. 12. The samples were examined Optical Metallography using an FEI Tecnai GS at an accelerating voltage of 200 kV.

The optical microscopy images are shown in Figs. 14–16. TEM Data Table 8 summarizes the observations made for Figs. Extrudable

14–16, and it contrasts the microstructural feature with the The TEM images that were obtained to contrast the Al-Si-MG–Hall–Heroult mechanical properties. 12- year-old rivets with the new rivets are shown in

(a) (b)

10 μm 10 μm

Fig. 14 Sample 1 (12 years old): (a) Central section of mandrel (Al 7075), showing non-equiaxed grains and randomly located MgZn2 precipitates (dark regions); (b) typical section of sleeve (Al 5056), showing elongated grains indicative of 1016 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

(a) (b)

10 μm 10 μm

Fig. 15 Sample 2 (12 years old): (a) Central section of mandrel (Al 7075); (b) typical section of sleeve (Al 5056), showing elongated grains with pronounced flow, indicating plastic flow

(a) (b)

10 μm 10 μm

α Fig. 16 Sample 3 (new): (a) Central section of mandrel (Al 7075), showing irregular -Al grains and MgZn2 precipitates (dark regions); (b) typical section of sleeve (Al 5056)

Table 8 Summary of characteristics of rivet mandrels and sleeves Figure/ Grain Grain width sample Alloy Age length (μm) (μm) Microstructure VHN YS (MPa)a UTS (MPa)a 14(a)/1 Al 7075 12 years old 30–50 3–10 Irregular 184 491.3 582.4 14(b)/1 Al 5056 12 years old 5–20 3–10 Elongated 111 N/A N/A

Al-Si-MG–Hall–Heroult 15(a)/2 Al 7075 12 years old 30–70 10–35 Irregular 181 482.5 573.8 15(b)/2 Al 5056 12 years old 2–15 2–10 Curved grain flow 108 N/A N/A Extrudable Extrudable 16(a)/3 Al 7075 New 15–50 5–40 Irregular 168 444.5 536.8 16(b)/3 Al 5056 New 5–20 3–10 Elongated 104 N/A N/A aUsing conversion according to Pagliarello.[30]

Figs. 17 and 18. Unit scales for images are shown as follows: observed. This suggests that any diffraction contrast aris- a = 500 nm, b = 200 nm, and c = 500 nm, respectively. ing from the precipitates was too weak to be present in the SAED pattern. TEM Observations Examination of the mandrel section in the new rivet (Sample 3) revealed a microstructure that was broadly Examination of the mandrel section of 12-year-old rivets similar to that of the 12-year-old rivets. It consisted of fine (Sample 1) revealed a microstructure comprising a disper- nanoscale precipitates together with a dispersion of coarser

sion of Mg-Zn-rich particles (MgZn2 precipitates), which particles. The SAED pattern was essentially identical to probably are η′. These features are consistent with the that of the other rivet. microstructures observed by Li et al.[39] in Al 7075 sam- More detailed examination of the precipitate distribu- ples aged at 120°C for 24 h. The selected area electron tion in the 12-year-old rivet suggests that there is a small diffraction (SAED) pattern recorded from the <001> alu- decrease in the size of the coarser Mg-Zn-rich particles. minum zone axis showed, as expected, strong reflections Comparison of the TEM images suggests that in the new from the aluminum matrix; no additional reflections or rivet, these coarser particles are slightly larger than ∼50 diffraction contrast from the second-phase particles was nm, whereas, after the in-service lifetime, these particles Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1017

(a) (b) (c)

Fig. 17 Sample 1 (12 years old): (a) Bright-field TEM image in mandrel section, showing grain boundary and MgZn2 precipitates ∼ ( 25–50 nm) throughout grains (dark spherical and rodlike); (b) bright-field TEM image, showing MgZn2 precipitated mostly within grains (dark spherical and η’ rodlike); (c) selected areas of electron diffraction pattern oriented parallel to <001> axis of Al matrix

(a) (b) (c)

∼ Fig. 18 Sample 3 (new): (a) Bright-field TEM image in mandrel section, showing MgZn2 precipitates ( 50–100 nm) throughout grains

(dark spherical and rodlike); (b) bright-field TEM image, showing MgZn2 precipitated mostly within grains (dark spherical and rodlike); (c) SAED pattern oriented parallel to <001> axis of Al matrix are slightly smaller than ∼50 nm. The particles in the From Table 8 in “Optical Metallography” section, the 12-year-old rivet also appear to be slightly more spheri- summary of the grain size and Vickers microhardness indi- cal than those in the new rivet. These observations are cate that the Al 5056 sleeve material experienced mechanical consistent with some reversion of the precipitate particles working since the grains were elongated. The average micro- over time as observed in the 12-year-old rivets. In contrast, hardnesses of the 12-year-old Al 5056 sleeve were Sample there were no obvious differences in the size or distribution 1 111 Hv and Sample 2 108 Hv, whereas the average micro- of the η′ particles between the samples representing the hardness of the new sleeve was 104 Hv, giving a difference of 12-year-old rivets and new rivets. 4–7 Hv and a decrease of ∼4%–7%. In contrast, the average As shown in Fig. 17a,b, the TEM image of the 12-year- microhardnesses of the 12-year-old Al 7075 mandrel were old rivet included a grain boundary. It can be seen that Sample 1 184 Hv and Sample 2 181 Hv, whereas the aver- Extrudable there is no evidence of PFZs or enriched grain boundary age microhardness of the new mandrel was 168 Hv, giving a precipitation. greater difference of 13–16 Hv and a decrease of ∼7%–10%. Al-Si-MG–Hall–Heroult Although the reason for the differential in the microhardness decreases of the two alloys is unknown, it is probable that DISCUSSION this derives from a combination of the different character- istics of the alloys and the stress and corrosion conditions to The main interest of the present work lies in its examina- which they were subjected. tion of the effect of long-term environmental aging of aero- Optical microscopy revealed that the Al 5056 sleeve space rivets used for construction purposes. The conditions exhibited elongated grains owing to rolling or grain flow of exposure had the potential to be relatively severe in that around the curved section near the rivet head, both of they involved cyclic heating from sunlight, irregular wind which are indicative of plastic flow during the loading, exposure to rainwater, and sea spray. Hence, the process. The grain shapes and sizes of the 12-year-old rivets would have been subjected to various stresses such and new sleeves were essentially identical, so it is not sur- as tensile, compressive, and shear as a result of the wind prising that microhardnesses are similar. Figs. 14a, 15a, loading acting upon them. Furthermore, the conditions for and 16a represent the mandrel of Al 7075 for samples corrosion clearly were present, and there does not appear 1, 2, and 3, respectively. Optical microscopy shows that to be equivalent data in the literature. the 12-year-old mandrels (Figs. 14a and 15a) exhibited 1018 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

grain lengths in the 30–70 μm range and widths in the found that the precipitate size in solutionized Al 7075 3–35 μm range. However, the new mandrels (Fig. 16a) actually increased to 1.2 nm after natural aging for 25 exhibited grain lengths of 15–50 μm and widths of 5–40 years according to Hatch.[41] In this study, there was a μm. In effect, the new mandrels appear to consist of nearly corresponding YS increase from the quenched condition, equiaxed but larger grains. increasing from 150 to 465 MPa, the latter of which is In the TEM images, the dark regions represent- nearly that is achievable in the T6 condition.[41] The GP

ing MgZn2 inclusions are variable in size and are in the zone precipitate size in the T6 condition generally is in the cross-section range ∼1–5 μm. Their distribution is rel- 2.0–3.5 nm range, and the microstructure also contains atively uniform and essentially the same for both the traces of hexagonal metastable phases which is rich in Zn 12-year-old (Figs. 14a and 15a) and new (Fig. 16a) man- and Mg. These features are generally consistent with those drels. Most of these precipitates in the mandrel sections of of the present work for both the 12-year-old and new riv-

the 12-year-old rivets generally are spheroidal (see Figs. 17 ets. However, in the present work, the MgZn2 precipitates and 18), with some rod-shaped precipitates also present. are larger at 50–100 nm, although there is also a disper- The relatively uniform distribution of precipitates suggests sion of finer precipitates in the 1–3 nm range. Concerning the homogeneous nucleation of Guinier–Preston (GP) zone the dispersion of finer precipitates as Hatch describes,[41] precipitates, which could provide solute strengthening. this is only comparable with the nanoscale precipitates There also may be a slight hardening effect during aging observed in both the new and old rivets evidencing the T6 due to a reversal process. treatment. The TEM images show that the new rivets contained Therefore, the presence of the dispersion of finer pre- nanoscale η′ precipitates as well as coarser Mg-Zn-rich par- cipitates in both the 12-year-old and new rivets suggests ticles that were approximately in the few tens of nanometers that they were treated in the T6 condition. If so, then the in diameter. These microstructural characteristics are sim- present work reveals that the principal physical and micro- ilar to those reported by other researchers[39] for Al 7075 structural changes are reflected in the decrease in the aged at 120°C for 24 h. The 12-year-old rivets revealed a size of the precipitates, which resulted in a correspond- microstructure that was similar to that of the new rivets in ing increase in microhardness of the mandrels, and the that there were no PFZs or enriched grain boundary pre- absence of corrosion. cipitation. The only major difference is that the 12-year-old It appears that the precipitates are larger in the new rivets rivets exhibited precipitates (Fig. 17a,b) that were slightly when contrasted to the precipitates observed in the 12-year- smaller than those of the new rivets (Fig. 18a,b). This prob- old rivets. It would be misleading though, to compare the ably is due to slight variations in production compositions new rivets with the 12-year-old rivets and then argue that and conditions. the older rivets became harder by such a large difference As cyclic exposure to heating from sunlight was a due to aging kinetics occurring as a result of being exposed component of the aging process, an interesting point is to the outdoor temperature over the 12-year period. Also, it whether such aging may take place in the future. This is not necessarily incorrect to infer from this, however, that

Al-Si-MG–Hall–Heroult appears to be a distinct possibility since it is known that natural aging is definitely not a contributing factor for the the kinetics of aging of Al 7075 apply over many years, increase in hardness and strength. Extrudable Extrudable even at room temperature.[40] Furthermore, the disso- It would be expected that the substantial gains in YS lution rate of the precipitates may be a relevant factor, (34.8–46.8 MPa) and UTS (23.8–45.6 MPa) would be albeit probably at a very slow rate. These issues impact reflected in microstructural changes, although these on the potential for further hardening and strengthening appear to be minimal. However, these changes are a direct and are important owing to the desirability of increasing reflection of the microhardnesses, which generally are both the mechanical properties and the corrosion resis- viewed as being strongly dependent on the microstruc- tance. The Al 7000 series alloys attain high strengths in ture. The properties of the coarser metastable η′ phase the T6 condition, but their corrosion resistance is poorest in the unused rivets may be compared to those of Vijaya in this condition.[39] Conversely, the corrosion resistance et al.[41] where it has been proposed that the coarsening of is increased when the alloy is overaged in conditions such the precipitates and its dissolution leads to a reduction in as T73, T76, or T74. hardness. Since the mandrels originally were subjected to a T6 This notion is substantiated by Hudson[37] where spec- temper, further substantial improvements in the mechani- imens of 7075-T6 aluminum alloys were compared to the cal properties would not be expected. However, it is clear properties in the same alloy some 19 years later, i.e., that the mandrels, rather than degrading, have obtained the first tests were conducted in 1949, and the later tests just such increases in the microhardness, YS, and UTS. were conducted in 1968. It was shown that there was a Both the optical microscopy and TEM images reveal slight change in tensile properties over time in service, and

that some dissolution of the MgZn2 in the mandrels has that there was a small increase in yield and UTS (5 and 2 occurred over time. Although most studies have reported MPa, respectively). A fatigue study also showed that there natural aging effects following solutionizing, a key study were negligible differences of fatigue performance of the Field Trials of Aerospace Fasteners in Mechanical and Structural Applications 1019 aluminum alloy when it was first tested in 1949 to when it During the installation of the rivet (see Fig. 19), from left was again tested later in 1968. The calculated change in the to right, the malleable hollow sleeve (dark gray) expands YS was attributed to natural age hardening kinetics. and forms around the contours of the mandrel blind head Although the hardness test can be a good indicator for (light gray bulge at the bottom of Fig. 19 far right) to take the tensile and YS of the material, materials exhibiting up any slack as the mandrel is pulled through the sleeve, defects such as corrosion or cracks will tend to reduce the and the sleeve tightly fills the hole that was drilled. On tensile strength of the material holistically. The assump- the opposite end of the blind head as shown in Fig. 19 tion behind the generalized empirical equation is that the (last two on the right side), the mandrel snaps once the given material must be reasonably homogeneous from a tensile strength of the mandrel alloy can no longer over- structural or solid mechanics perspective, because mate- come the resisting force securing the blind head in place rial defects will create stress concentrations leading to fail- against the pieces being riveted. This clamping action is ure below expected tensile strength values for the given what allows the relatively stronger 7075 mandrel to shape material. However, given that the hardness results are or extrude the more malleable 5056 sleeve so that the end is typical and agree well with the literature, visual, micros- plugged and prevents ingress of water, chlorine, and other copy, and TEM analysis further provided evidence that air-borne contaminants. The level of extrusion in the sleeve corrosion is not at all present, hence it is suggested that the while being formed in shape around the blind head is also rivets are corrosion free. Hardness tests on slightly cor- subject to work hardening. The wind loading on the struc- roded (very small amounts of corrosion) 7075 specimens ture is also continually creating to some minor degree ten- have been found to yield significantly much lower hard- sile, compressive, shear, and tribological action upon the ness and lower UTS values than the expected hardness sleeve. However, the level of work hardening here may also readings as per Obert et al.[32] The hardness results for the be minute, but difficult to qualify at this stage in the project. rivets in this article therefore were not lower, but rather There is little issue with galvanic corrosion between the high when comparing to most aluminum alloys, and they mandrel and the sleeve as they are very close in the galvanic were in agreement with the results in the literature. This series and on the anodic side, since the solution potentials validated almost with complete certainty that no corrosion of the aluminum alloys 1000, 3000, 4000, 5000, 6000, and could have been present on the rivets surface nor within the 7000 series are anodic, whereas 2000 series is cathodic. subsurface regions. Furthermore, the rivets were coated with water-based paint The literature seems to shy away from case studies on the side of where the mandrel snaps which provided or details regarding sleeves of rivets, and the sleeves are further sealing. The 5000 series alloys have excellent resis- no less important to the rivet structure than the mandrel tance to corrosion in a marine environment, except when itself. Basically, the integrity of the sleeve is what holds subjected to temperatures in excess of 60°C, and alumi- the entire rivet together and is the direct interface holding num in general is resistant to corrosion when subjected to the structure in place, since we have seen directly what a pH range between 4.5 and 8.5 in comparison with steel, happened to the failed high-rise building roof structure in which would corrode at approximately 2–3 times that of 2003 when the failed rivets (made from steel mandrel with aluminum in the same pH range. Generally, aluminum aluminum sleeves) were assessed. The poor design for used in aerospace or external surfaces of buildings will be these original rivets in the building, meant that the steel exposed to rain, and any accumulated salt on aluminum mandrel which inevitably corroded leaving intact only the surface will be easily washed off, as is the case for these

aluminum sleeve, did not have the required strength to rivets since they are continually exposed to rain. How- Extrudable hold up the structures, and this was one of the main rea- ever, the main disadvantage with aluminum with respect sons for the catastrophic failure. Rivets are therefore ren- to electrolytes would be excessive exposure to higher than Al-Si-MG–Hall–Heroult dered futile once either of the components—mandrel or normal temperatures, but this was not a major concern for sleeve—deteriorates, and they must act together to fulfill the authors with aluminum in the outdoor environment, as the requirements. these temperatures are rarely experienced in Australia.

Fig. 19 Sequence of the rivet installation from left to right. The hollow sleeve expands and forms around the mandrel blind head 1020 Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

ACKNOWLEDGMENTS 12. Videla, H.A. Manual of Biocorrosion; CRC Press: Boca Raton, FL, 1996. The authors express their appreciation to the following 13. ASM Aerospace Specification Metals Inc. Available at individuals from the University of New South Wales Syd- http://asm.matweb.com. ney: Professor Alan G. Crosky for his guidance and assis- 14. MatWeb, Material Property Data. Available at http://www. matweb.com/search/DataSheet.aspx. tance with the sample preparation, Dr. George Yang for his 15. AZO Materials, Aluminium/Aluminum 7050 Alloy (UNSW technical assistance with the equipment in his laboratory, A97050); Available at http://www.azom.com/ article. and Mr. Jonathan Delley for his general assistance in the aspx?ArticleID=6650. laboratory. 16. Alcoa Fastening Systems, HUCK-CLINCH® Process Manual, 2005. 17. Huck Magna-Lok Fasteners as per Engineering and Instal- REFERENCES lation Standards; HWB-865S and HWB-865T; Huck International, Inc.: Waco, TX. 1. Polmear, I.; St John, D.; Nie, J.-F.; Qian, M. Light Alloys, 18. Moreira, P.M.G.P.; da Silva, L.F.M.; de Castro, P.M.S.T. Metallurgy of the Light Metals, 5th Ed.; Elseiver Ltd.: Structural Connections for Lightweight Metallic Struc- Amsterdam, 2017, 31. tures. Advanced Structured Materials; Springer-Verlag: 2. Mouritz, A.P. Introduction to Aerospace Materials; Wood- New York, 2012. head Publishing Limited: Oxford, 2012. 19. Rendigs, K.H.; Knwer, M. Metal materials in Airbus A380. 3. Baker, H.; Benjamin, D.; Cubberly, W.H.; ASM International In 2nd Izmir Global Aerospace and Offset Conference, Handbook Committee. ASM Committee on Aluminum Gaziemir-Izmir, 2010. and Aluminum Alloys. Introduction to aluminum and alu- 20. Van Vlack, L.H. Elements of Materials Science and minum alloys. In Metals Handbook, Volume 2: Properties Engineering ; Prentice Hall: Reading, MA, 1989, 534. and Selection: Nonferrous Alloys and Pure Metals, 9th Ed.; 21. Hatch, J.E.; Ed.; Aluminium. Properties and Physical American Society for Metals: Metals Park, OH, 1979, 28. Metallurgy; American Society for Metals: Metals Park, http://www.worldcat.org/title/metals-handbook-vol-2-prop- OH, Tenth printing, April 2005. erties-and-selection-nonferrous-alloys-and-pure-metals/ 22. Baker, H.; Benjamin, D.; Cubberly, W.H.; ASM Interna- oclc/840172564?referer=di&ht=edition. tional Handbook Committee. ASM Committee on Alumi- 4. Gale, W.F.; Totemeir, T.C. Smithells Metals Refer- num and Aluminum Alloys. Introduction to Aluminum and ence Book; ASM International. Elsevier Butterworth- Aluminum Alloys. In Metals Handbook, Volume 2: Prop- Heinemann: Oxford, 2004. erties and Selection: Nonferrous Alloys and Pure Metals, 5. Baker, H.; Benjamin, D.; Cubberly, W.H.; ASM Interna- 9th Ed.; American Society for Metals: Metals Park, OH, tional Handbook Committee. ASM Committee on Alumi- 1979, 62. http://www.worldcat.org/title/metals-handbook- num and Aluminum Alloys. Introduction to aluminum and vol-2-properties-and-selection-nonferrous-alloys-and-pure- aluminum alloys. In Metals Handbook, Volume 2: Prop- metals/oclc/840172564?referer=di&ht=edition. erties and Selection: Nonferrous Alloys and Pure Metals, 23. Baker, H.; Benjamin, D.; Cubberly, W.H.; ASM Interna- 9th Ed.; American Society for Metals: Metals Park, OH, tional Handbook Committee. ASM Committee on Alumi-

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